Human Manure

Returning fertility to farms and gardens

Abraham Noe-Hays

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Abraham Noe-Hays

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June 01 , 2010

In gardening, we cannot escape cycles—not that we’d want to, since they’re what keeps the whole party going. There are the obvious cycles, like the eternal cycle of seasons, and the accompanying growth cycle from seed to seedling, to plant, flower, or fruit, and back to seed again. But there’s another cycle taking place in every garden and on every farm that is the most fundamental of all, but nearly invisible.

This cycle is the constant flow of the chemical building blocks of life—elements such as nitrogen, potassium, phosphorous, calcium, magnesium, and so on. They’re the unliving constituents that plants absorb and then miraculously rearrange into vibrant, living, and delicious forms, such as the cantaloupe. These nutrients are always in motion, and in nature they flow in a cycle, from soil to plant to animal, and then back to soil again.

But on most farms and gardens the nutrient cycle is broken, and the unlikely culprit is your toilet. Allow me to explain.

As plants grow, nutrients flow into them from the soil, and become the raw material for the plant’s developing leaves and fruit. When we harvest plants and take them off the farm, we remove from the farm the nutrients those plants contain. We bring these nutrient-rich plants into our homes where we eat and digest them, extracting their energy and using their various elements throughout our bodies. Then, when those elements are no longer in a form we can use, we excrete them, typically into a flush toilet filled with potable water.

Some of the nutrients we flush pass right through septic systems and sewage plants to become pollution in rivers and lakes, where they can cause algal blooms and kill fish. Another portion ends up buried in the leach fields of septic systems, or lost as gaseous nitrogen into the air. The remainder of the nutrients end up in sewage sludge that goes into landfills or gets spread onto farmland. While it’s good, in principle, to see some of these nutrients being recycled back onto farmland where they can grow more food, the problem is that sewage sludge contains not just manure, but all the other toxic substances that people and industries flush down the sewers.

The problem, then, is simple: we are extracting nutrients from the soil, then flushing them down the toilet. Most of these nutrients are lost, while the remainder are often mixed with toxic chemicals, rendering them of questionable value. To replenish the depleted soil and to keep the food industry rolling, we spend vast quantities of money and energy to produce synthetic fertilizers. To keep the rivers from being overwhelmed by the enormous volume of flushed manure, we spend even more to partially clean up the water we have dirtied. This is the height of unsustainability.

But this is not a story of doom and gloom. Although this system is broken, probing the nature of this break, and ultimately mending it, is a process that is both fascinating and rewarding. And the solution is not a pipe dream, based on untested theories—rather, it is rooted in experience that stretches back at least 4,000 years, when the ancient Chinese first learned to close the cycle by returning human manure to agriculture.

Like any other animal manure, human manure is a powerful fertilizer. If we could capture and reuse all our manure, the benefits would be tremendous. Chemical analysis reveals that an individual’s yearly output of manure contains enough fertilizer to grow between 50 and 100 percent of the food that person needs to eat. Put differently, the global output of human manure contains enough nutrients to grow anywhere from 50 to 100 percent of the food required to feed the world. The wide range in numbers reflects the variability of real-world conditions, but either way, the conclusion is profound. Recycling our manure not only saves water and prevents pollution, but also lets us slash synthetic fertilizer production, an energy-intensive process that depends on non-renewable resources like natural gas and rock phosphate. As a substitute for synthetic fertilizers, human manure is a vast, sustainable, and inherently local resource that can increase food security and regional self-sufficiency.

There are several challenges in reusing human manure. The most obvious is how to make it hygienic. Another issue is how to turn manure into something pleasant to handle and useful to plants. Composting toilets are designed to do both these things, destroying pathogens while creating a valuable compost. They come in many shapes and sizes, but at their heart they all have a container that gradually fills with manure and a dry material, such as wood shavings, and contains them as they decompose.

Properly designed composting toilets are odorless and can be used in homes, schools, and businesses. They typically use a small electric fan to constantly pull air out of the compost chamber and exhaust it through the roof, which prevents any odors from migrating from the compost into the bathroom. Most high-capacity systems have a toilet-size commode in the bathroom, connected to a composting container on the floor below, although some light-duty units fit entirely inside the bathroom. A third approach is to use a commode that houses a small container that is emptied frequently into an outdoor compost bin.

A well-designed composting toilet creates an ideal environment for decomposer organisms to break down manure. Just like any of us, these organisms have certain essential needs: oxygen, water, an agreeable temperature, and a balanced diet. The wood shavings (or other dry, cellulose-rich organic matter) help with the diet, adding carbon to the compost and thereby balancing all the nitrogen in the manure. They also absorb moisture and give structure to the compost, creating air spaces that allow oxygen to diffuse through the compost pile. Temperature affects how quickly the decomposers will work. If the compost stays at room temperature or higher, decomposition is fast, while below 50°F it slows to a crawl. Moisture must remain at a happy medium. Too much moisture fills air spaces, blocking the flow of oxygen. Too little, and the toilet becomes a desert, stopping decomposition in its tracks.

Over time, composting toilets eliminate pathogens. During the decomposition process, any disease organisms in the compost find themselves outside their preferred environment (your body) and forced to fight a losing battle against all the robust compost organisms that inexorably eat them for lunch. What is left at the end is a crumbly, dark humus with the aroma of rich soil. Although guidelines vary, many recommend aging the compost 12 to 24 months before using it. As an additional health safeguard, compost can be kept out of contact with the edible portion of crops by using it beneath fruit trees or incorporating it underground. Some states, including Vermont, have specific requirements that differ from these generic recommendations.

Historically, most composting toilets have combined both forms of manure, pee and poop, together in the same container. Although this approach works, a more refined method is gaining popularity. This is the technique of urine diversion—keeping urine and feces from ever mixing, and treating them each according to their own very different properties. The advantages are many. Keeping urine out of the compost makes it easier to control moisture, accelerates decomposition, and keeps the compost container from filling so fast. But best of all, it allows for the collection of pure urine, which turns out to be a spectacular resource. Urine may seem like little more than water, but in reality it contains as much or more phosphorous and potassium as the solids, and five to ten times as much nitrogen. In addition, urine from healthy people is nearly sterile, and even when bacteria are present, they die within days when urine is stored outside the human body. So with most of the nutrients and none of the problematic pathogens, pure urine is the most powerful and safest portion of our manure, requiring no treatment other than a brief storage period.

Using urine as a fertilizer is the easiest way to begin closing the cycle and returning the nutrients in our manure to the soil. All it requires is a way to catch and store urine, and a farm or garden on which to use it. Suitable for growing all manner of grains and vegetables, it provides a dramatic boost to heavy nitrogen feeders, and is particularly good for giving plants a strong start in the spring, when soil nutrients are still bound up in the cool ground. Application is easy—common practice is to dilute one part of urine with three or four parts of water, and then water it directly into the ground around plants. One person’s yearly output of urine is enough to thoroughly fertilize 1,500 square feet—that’s a big garden!

In this country, regulations currently limit the widespread reuse of urine, but the story is different elsewhere. Numerous university studies, particularly in Scandinavia, have established the safety of urine for fertilizing vegetables, and have demonstrated that its nutrient value is on par with commercial fertilizers. The Swedish EPA and the World Health Organization are both developing guidelines for its use, recognizing that recycling urine not only benefits the environment, but can also improve human health in places where lack of affordable fertilizer results in inadequate diets.

What drives me in my work building composting toilets throughout New England, as well as in Central America with the nonprofit organization Sustainable Harvest International, is the desire for a world where agriculture is truly sustainable, and where humans are an integral part of the nutrient cycle. Getting to that point is challenging, and involves updating the way we relate to our own manure, but the vision is compelling: when every toilet becomes a fertilizer factory, we will put an end to the waste and pollution that result from squandering our manure as trash, and reap the abundance that comes from treating it as a treasure.

Photo courtesy of Abe Noe-Hays

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About the Author

Abraham Noe-Hays, of Putney, is the owner of Full Circle Compost Consulting, which designs, manufactures, installs, and services composting toilets throughout the northeast. He holds a B.A. in human ecology from College of the Atlantic, which now boasts new dormitories and a student center containing composting toilets. For more information about human manure, and for recommendations for further reading, visit Abraham’s website.

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